The proposed research is addressed to a central debate in research on the causes of fertility transition, namely the role of diffusion processes. Th research will examine this issue using Costa Rica and Taiwan as case studies. The principal objectives of the research are: Conceptualization of the nature of diffusion effects, with particular attention to their relationship to the effects of social and economic variables. Development of approaches for empirical investigation of diffusion effects. Analysis of diffusion effects on fertility regulation behavior in Costa Ric and Taiwan for the periods from 1961-62 - 1980. During the periods under consideration, Costa Rica and Taiwan experienced extremely rapid fertility declines, largely the consequence of adoption of modern contraceptive methods by married couples. This adoption process wil be modeled as a function of social and economic conditions and of diffusion effects, using detailed aggregate-level data (81 cantons in Costa Rica, 361 townships in Taiwan) and a series of national fertility surveys conducted in each country. Extensive information on fertility, social and economic variables, and geographic location is available at both the aggregate and the individual level. The analysis will take place in three stages: Descriptive analysis at the aggregate level of the spatial-temporal pattern of fertility change. Statistical analysis at the aggregate level of the spatial-temporal pattern of fertility change, taking into account social and economic variables. Multilevel analysis in which the contraceptive behavior of individual women is modelled as a function of individual and areal variables. In the second and third stages, a basic feature of the models examined will be the inclusion among the explanatory variables of indicators of the level of fertility control in other areas.